In a principal aspect the present invention relates to a cable drum construction for a multiple panel garage door lift assembly. Garage doors, industrial doors and other closures for building openings may include multiple horizontal panels which are hinged together and collectively mounted on a track to enable movement of the multiple, pivotally connected panels between an open position and a closed position. To facilitate movement between and an open and a closed position, a motorized lift assembly or mechanism is typically provided. The lift assembly may include a spring biased cable arrangement which facilitates movement of the multiple, articulated, horizontal panels between the open and closed positions along a track.
Such lift assembly constructions or mechanisms thus may include a cable affixed to a lower panel of a door and wound on a drum attached to a spring biased torsion rod mounted on the frame or header for the door. The design of these constructions is well-known and the design of cable drums associated with such constructions has been the subject matter of various known developments including the disclosure in U.S. Pat. No. 7,343,958 issued Mar. 18, 2008 entitled Overhead Door Lift System, incorporated herewith by reference.
While cable door lift assembly designs have evolved over time, there remains a problem that has not been specifically addressed heretofore. That is, multi-panel doors conventionally comprise horizontal, hinged panels wherein all panels are of generally equal size, design and weight. For example, four identical panels may be arranged in an articulated or hinged array resulting in equal distribution of weight. Such constructions can be easily balanced for full travel along a track. However, often the first turn of a cable drum for such a multi-panel door includes a greater radius designed to decrease the initial spring lifting force and thereby make the closed door heavier and consequently assist maintaining the door in a vertical, fully closed position.
However, with many doors the top panel or section is heavier and as a consequence, the multiple sections of the door do not balance properly because torque per turn of the biasing door support spring is constant but the rate of change of the door weight is not constant. For example, the top section may be glazed, that is, include window panels incorporated therewith. Also the top panel may have a height dimension which is greater than the remaining door panel. Such glazing or extra height may cause a top panel to be heavier than the remaining sections or panels of the door. Therefore known prior art designs are not suitable to properly balance a door that comprises a heavier top panel or section. Further, once that top or heavier panel section rises into the horizontal position on the door track, that panel becomes less consequential with respect to movement of the entire assembly of door panels to the fully opened position. Thus, to accommodate an arrangement for lifting which includes a heavy top panel of a multi-panel door, is a problem that faces manufacturers of such panel doors.